Is Free Cyclooctatetraene Dianion an Aromatic System? A Quantum Chemistry Study

To investigate whether free cyclooctatetraene dianion (COT2−) is aromatic, quantum chemistry methods were used to optimize its structure. Based on the optimized structures, the natural population analysis (NPA) charge, bond order, delocalization energy, nucleus‐independent chemical shift (NICS), and harmonic oscillator model of aromaticity (HOMA) values were computed by DFT‐B3LYP method with basis set 6‐311++G**, which shows that COT2− is not aromatic as it is not planar and has different bond lengths and bond orders, smallest delocalization energy and positive NICS values. To further confirm the finding, the changes of NICS and energy against ring distortion angle were scanned. The COT2− has positive NICS values all along the angle from 180° to 120° while other aromatic systems always have negative values. The energy scanning suggests that COT2− should have the weakest capability to maintain its planar structure. All the calculations strongly indicate that COT2− is not aromatic. This study also suggests that NICS scan might be a good approach to judge aromaticity. To investigate whether free cyclooctatetraene dianion (COT2−) is aromatic, quantum chemistry methods were used to optimize its structure, then predicted the NPA charge, bond order, delocalization energy, NICS, and HOMA values at B3LYP/6‐311++G** level. The calculation results showed that free COT2−, which has D2d symmetry and is the lowest energy conformation, is not aromatic.